1. Field of the Invention
The present invention pertains to optical communication, optical interconnects, and optical signal processing employing semiconductor lasers. More particularly, the present invention relates to a monolithic multi-wavelength semiconductor laser unit for outputting laser lights having different wavelengths from a single monolithically integrated semiconductor chip, which can be used in optical memory disk applications.
2. Description of the Related Art
An optical memory disk system is widely put to practical use since it is small-sized and it can record a large capacity of information. Especially, in a digital versatile disk (DVD) system, its practical use has been rapidly promoted as a main system such as a movie, a multi-media application for a next generation and the like. On the other hand, a compact disk (CD) system or a compact disk-recordable (CD-R) system has been conventionally widespread as an optical memory disk. It is desirable that the DVD system has compatibility with the CD system. That is, it is necessary that the DVD system can read and write data from and to a disk of CD or CD-R. Those optical memory disk systems employ an optical pickup using a semiconductor laser, in order to read out the information recorded on the disk, and/or write the information on the disk.
FIG. 1 is an explanation view showing a typical configuration proposed as an optical pickup of a conventional DVD system. That is, the optical pickup in FIG. 1 has compatibility with the disks of CD and DVD, and has an optical integration unit 1 for DVD and an optical integration unit 2 for CD and CD-R. A laser light having a wavelength of 650 nm emitted by the optical integration unit 1 for DVD passes through a dichroic prism 3, and also passes through a collective lens 4, a beam-rising mirror 5, a wavelength selection diaphragm 6 and an objective lens 7, and then reaches an optical memory disk 9. On the other hand, the prism 3 reflects a laser light having a wavelength of 780 nm, emitted from the optical integration unit 2 to the recording surface of CD. Then, it passes through an optical path substantially equal to that of the laser light emitting the radiation with the wavelength of 650 nm for DVD, and reaches a CD or CD-R disk 8. And, the return lights from the disk pass through optical paths opposite to the above-mentioned paths, and reach the optical integration units 1, 2 for DVD or CD, respectively.
However, the conventional optical pickup uses the two different optical integration units 1, 2 in order to obtain the laser light with the wavelength of 650 nm and that with the wavelength of 780 nm. Thus, its configuration becomes complex which results in a problem that it is difficult to make the optical pickup smaller and lighter. Also, the finely positional adjustments for respective light sources must be done to thereby require a very long time for assembling the optical system.
In order to solve such problems, a multi-wavelength semiconductor laser unit is proposed which can independently output laser lights having two different wavelengths of 650 nm and 780 nm from .one chip, as shown in FIG. 2A, which is proposed by Uchizaki et al. in Japanese Published Unexamined Patent Application No.P2000-11417A (hereinafter referred as xe2x80x9cUchizaki et al.xe2x80x9d). The semiconductor laser in FIG. 2A radiates the laser lights having the two different wavelengths. So, two active regions are arranged in parallel to each other in the optical axis directions. In FIG. 2A, the laser structure referred to as xe2x80x9ca selectively buried ridge (SBR) structurexe2x80x9d is shown. That is, p-type InGaAlP cladding layers 109, 119 are formed in ridge geometries. Then, both sides of the ridge are sandwiched by n-type GaAs layers 123 having column V elements Arsenic (As) different from the column V elements Phosphorus (P) in the cladding layers 109, 119. The n-type GaAs layers serves as current-blocking regions to thereby channeling current into active layers. At the same time, the GaAs layer in which band gap is narrower than that of the active layer absorbs a light transmitted through the active layers in the lower portions, at both the sides of the ridge 109, 119.
In detail, even in any of laser portions 100, 101, n-type buffer layers 102, 112, n-type InGaAlP cladding layers 103, 113, InGaAlP waveguide layers 104, 114, multi-quantum well (MQW) active layers 105, 115, InGaAlP waveguide layers 106, 116, first p-type InGaAlP cladding layers 107, 117, p-type InGaP etching stop layers 108, 118, second p-type InGaAlP cladding layers 109, 119, p-type InGaP conduction layers 110, 120, n-type current-blocking layers 123 and p-type GaAs contact layers 122 are laminated on substrates 124 in this order.
Here, in the first laser portion 100 emitting the radiation with the wavelength of 780 nm, the active layer 105 has the MQW structure composed of Ga0.9Al0.1As quantum well layer and Ga0.65Al0.35As barrier layer. In the second laser portion 101 emitting the radiation with the wavelength of 650 nm, the active layer 115 has the MQW structure composed of In0.5Ga0.5As quantum well layer and In0.5(Ga0.5Al0.5)0.5P barrier layer.
That is, the active layer 105 emitting the radiation with the wavelength of 780 nm and the cladding layers 103, 107 and 109 have column V elements different from each other, namely, P and As. Also, the active layer 41 emitting the radiation with the wavelength of 650 nm and the cladding layers 22, 26 and 28 have column V elements P common to each other. This structure enables the compositions and the film thicknesses of the cladding layers 103, 107 and 109, and 113, 117 and 119 in the devices 100, 101, the compositions and the film thicknesses of the high conductivity films 110, 120, the compositions and the film thicknesses of the current-blocking layers 123 and the compositions and the film thicknesses of the contact layers 122 to have the commonalities to each other, and also enables the manufacturing processes to be very easy and further enables the control accuracies to be very high.
However, this also brings about a problem. FIG. 2B describes this problem. In FIG. 2B, the compositions, the carrier concentrations and the film thicknesses of the respective semiconductor layers are plotted, from the n-type cladding layer 103 to the p-type third cladding layer 109, along the lamination direction when a forward bias of 2.5 V is applied across the electrodes. The film thicknesses of the respective semiconductor layers are considered to accordingly simulate the energy band diagram, the Fermi-level diagram and the distribution of electron current densities. In the n-type cladding layer 103, the Al mole fraction is 0.7 as In0.5(Ga0.3Al0.7)0.5P, the carrier concentration is 2xc3x971017cmxe2x88x923 and the layer thickness is about 1 xcexcm. The compositions of the waveguide layer 104 and the buffer layer 102 are defined as undoped In0.5(Ga0.5Al0.5)0.5P. The active layer 105 is a double quantum well (DQW) composed of two quantum well layers, namely, a first quantum well layer in contact with the n-type cladding layer 103 and a second quantum well layer adjacent to a right side of the first quantum well layer. Each of them is assumed to have the film thickness of 10 nm and the composition of undoped Al0.1Ga0.9As. There are the two p-type cladding layers 107, 109 sandwiching the etching stop layer 108 between them. However, they are designed such that all of the compositions are equal to that of the n-type cladding layer 103, the carrier concentrations are 1xc3x971017cmxe2x88x923 and the total thickness is equal to that of the n-type cladding layer. A longitudinal length of a resonator is defined as 600 xcexcm.
In a conduction band edge in FIG. 2B, there is a large peak reaching 45% with respect to a depth of the first quantum well layer, at a boundary between the first quantum well layer of AlGaAs and the waveguide layer 104 of InGaAlP in contact with the n-type cladding layer 103. This is a phenomenon referred to xe2x80x9ca band gap discontinuityxe2x80x9d, which is induced when semiconductors having the energy band gaps largely different from each other are in contact with each other. In the case of FIG. 2B, it results from a fact that the InGaAlP energy band gap (about 2 eV) is largely different from the AlGaAs band gap (about 1.6 eV). Also, because of the same reason, an energy level difference between respective bottom portions of the first and second quantum well layers is about 40% with respect to the depths of the first and second quantum well layers. So, the heights of the energy levels are extremely different. This difference brings about the situation referred to as a so-called xe2x80x9cnonuniform injectionxe2x80x9d, namely, the electron current injected into the active layer is not uniformly injected into the two quantum well layers. The band gap discontinuity and the nonuniform injection cause the injection efficiency of the electron to be very low. In the above-mentioned case, the electron current density in the active layer becomes 280 A/cm2. It does not satisfy several hundred to several thousand kA/cm2, which is a density necessary for a laser oscillation.
On the other hand, in the active layer 115 of the laser portion 41 emitting the radiation with the wavelength of 650 nm, the barrier layer of In0.5(Ga0.5Al0.5)0.5 P is made of compound semiconductor having the same component as the InGaAlP waveguide layer 114. Thus, this does not bring about the problems of the band gap discontinuity and the nonuniform injection. The laser having the normal wavelength of 780 nm or 650 nm made of the compound semiconductor of the same component can obtain a laser output of about several mW if there is an applied voltage of about 2.5 V. So, it is possible to understand the low injection efficiency with the structure shown in FIG. 2A. The result, when a laser having a resonator length of 440 xcexcm is actually fabricated under the above-mentioned structure, indicates that a voltage at which the laser oscillation is excited is equal to or greater than 2.7 V. Also, the voltage at which an optical power of 5 mW can be obtained is equal to or greater than 2.8 V. Those data agree with the tendency of the simulation result. If the laser is operating under such a high voltage, the power dissipation of a circuit for driving the laser is increased, which causes the rated specification, or the standard of the respective circuit elements to be strict, and accordingly makes the manufacturing cost higher, and further generates the heat-flow problem associated with the heat generation resulting from the increase of the power dissipation. This results in a severe problem on the circuit design.
The present invention is proposed to solve the above-mentioned problems. It is therefore an object of the present invention to provide a multi-wavelength semiconductor laser unit, which reduce a spike height at a band edge, or the band gap discontinuity induced at a boundary between a cladding layer and an active layer, and accordingly improves an operational voltage and an operational current.
Another object of the present invention is to uniform the electron currents injected into both active layers of first and second lasers by largely dropping the spike height caused by the band gap discontinuity at one of the laser in the multi-wavelength semiconductor laser unit.
Still another object of the present invention is to attain low power dissipation in a circuit for driving a laser by improving a carrier injection efficiency and dropping a voltage at which the laser oscillation is excited, in the multi-wavelength semiconductor laser unit.
Still another object of the present invention is to reduce a heat generation amount in a semiconductor chip by reducing a power dissipation, and then simplify a countermeasure for heat radiation, and accordingly make a design of a circuit for driving the multi-wavelength semiconductor laser unit and a package design easier.
Still another object of the present invention is to alleviate specifications of respective circuit elements used for driving circuit of the multi-wavelength semiconductor laser unit, and accordingly reduce manufacturing cost of the commercial products, such as a light source for an optical pickup in the multi-media instruments and the like.
In order to attain the above-mentioned objects, the feature of the present invention lies in a multi-wavelength semiconductor laser unit having a substrate, at least first and second lasers, which are merged on the substrate. The first laser has a bulk active layer, and the second laser has a MQW active layer. The term of xe2x80x9cthe bulk active layerxe2x80x9d is employed to distinguish from the MQW structure. In the bulk active layer, electrons can be treated by classical theories, but quantum mechanical analysis is required for the MQW structure. Here, the first laser emits a light of a first wavelength, and the second laser emits another light of a second wavelength different from the first wavelength. Also, the bulk active layer has a film thickness which is 0.01 xcexcm or more and 0.1 xcexcm or less. On the other hand, the MQW active layer is constituted by the stacked structure, or the super-lattice structure. Namely, the MQW active layer has a plurality of potential well structure, each of the potential well structure has a quantum well layer and a barrier layer adjacent to the quantum well layer. The second laser disposed on the substrate is isolated from the first laser by an isolation groove and the like.
In both the first and second lasers, an active layer and a semiconductor layer adjacent to the active layer constitute a double hetero-structure, respectively. This double hetero-structure generates a potential well of a conduction band edge at the active layer, and also enables a structure by which a carrier confinement is effectively attained. As stated above, in the multi-wavelength semiconductor laser unit of the present invention, the active layer of the first laser is configured to have the bulk active layer. Thus, it is possible to largely reduce a spike height caused by the band gap discontinuity at the hetero-junction boundary between the active layer of the first laser and the semiconductor layer adjacent to the active layer. Also, it is possible to flatten the bottom of the potential well at the conduction band edge formed in the active layer of the first laser. The spike height which is inherently generated by the band gap discontinuity at the boundary between the active layer of the first laser and the semiconductor layer adjacent to the active layer is largely reduced, by employing the structure of the bulk active layer. Hence, there is no problem of the nonuniform injection, which was known in the MQW active layer. That is, since the band gap discontinuity and the nonuniform injection are protected or suppressed in the first laser, the injection efficiency of the electron becomes very high. This results in a reduction of a voltage at which a laser oscillation is excited, and also leads to the low power dissipation of a circuit for driving the first laser. Also, the heat generation amount is reduced by the reduction of the power dissipation. Hence, the simple countermeasure for the heat radiation may be allowable to thereby make the circuit design easier. Moreover, the alleviation of the required standard or the rated specification for respective circuit elements enables the manufacturing cost to become lower.
Moreover, in the multi-wavelength semiconductor laser unit of the present invention, the film thickness of the active layer of the first laser is assumed to be 0.1 xcexcm or less. Thus, it is possible to protect a optical confinement coefficient xcex93 from being too large in the active layer of the first laser. Hence, it is possible to suppress the increase of an optical power density on the facet mirror. So, the deterioration on the facet mirror can be protected with regard to a operation for a long time. In short, this enables the achievement of excellent current injection efficiency and a high reliability.
As mentioned above, the multi-wavelength semiconductor laser unit of the present invention can output multiple-wavelength laser lights, each light having a single transverse mode, a small astigmatism, with lower operational voltages, lower operational currents, and an excellent productivity. Moreover, the multi-wavelength semiconductor laser unit of the present invention is suitable for the light source for driving the optical pickup that uses a wavelength different from that of CD-ROM, DVD-ROM or the like.